Devices that can sense departures of their own reference frame from an “inertial reference frame” are of important practical and commercial use—for example, in the area of inertial navigation and guidance. A gravity gradiometer is one such device, which is used to measure the gradient of a gravitational field.
Some gradiometers in the prior art utilize a main sensing element. This sensing element is a metallic sphere with high-density weights located at opposite ends of a diameter. The sensing sphere or float is symmetric about this diameter. It is suspended both electrostatically and by a fluid within a hollow sphere, and is free to rotate inside. When a given mass is passed by the device, the mass pulls more on the nearer weight than on the farther. A torque is thereby exerted on the sensing sphere, and the device then exerts a counter torque just sufficient to stop the motion of the sphere. The amount of counter torque is determined by a feedback system that is calibrated to a source of electric current. Since the moments of inertia of the sensing sphere are precisely known and the counter torque is measured, the components of the gradient of the gravitational field can be inferred. This type of gradiometer, however, has its limitations which include lack of portability and rapidity.
Some other gradiometer systems in the prior art utilize an orthogonal triad of gradiometers, where each gradiometer is assigned to a different directional axis, is independently rotatable, and comprises multiple pairs of accelerometers. The accelerometer pairs that constitute a particular gradiometer are rotated around that gradiometer's axis. Any difference in the acceleration measurements between the accelerometers in a pair translates to a gravity gradient, and measurements from multiple pairs of accelerometers are combined to produce a gradiometer output. Each gradiometer of this configuration measures two of the five independent components of the total gradient tensor. Measurements from the three gradiometers are necessary to measure the entire gradient tensor.
From a hardware perspective, the gradiometer system described immediately above requires rotating six (or more) accelerometer pairs around three orthogonal axes, consequentially requiring multiple motors, bearings, and slip rings. Because of the complement of hardware required, it is relatively costly to engineer a low failure rate with high signal-to-noise performance for this type of gradiometer system. Additionally, the accelerometers that are utilized are typically under-damped, thereby making the gradiometers very fragile and sensitive to small disturbances in the rotation rate.
What is needed is a gradiometer system that is capable of detecting relatively small changes in a gravitational field, without some of the costs and other disadvantages in the prior art.